1. Field of the Invention
This invention relates to a system for purifying an aqueous solution of crude caprolactam, especially by a system using an ion-exchange resin apparatus.
2. Description of Related Art
Caprolactam is industrially produced by, for example, a process including subjecting cyclohexanone oxime to an oleum-catalyzed Beckmann rearrangement reaction to form caprolactam; neutralizing the resulting reaction mixture containing oleum and caprolactam with ammonia water; and separating caprolactam from the formed ammonium sulfate. Usually, the separation is achieved, first, by extracting the mixture with an organic solvent, and then extracting the caprolactam-containing organic phase with water to obtain an aqueous solution of crude caprolactam, which contains about 30% to 40% of caprolactam.
Because caprolactam is an important starting material for synthesis of polyamide, the aqueous solution of crude caprolactam should be further purified to meet the high purity standards for caprolactam of industrial grade. So far, a variety of purification methods for obtaining caprolactam with high purity have been proposed, for example, purification by recrystallization, ion exchange by using an ion-exchange resin, distillation, etc. Among these methods, the method using ion-exchange resin is preferable, because it is very effective in removing ionic impurities from the aqueous solution of crude caprolactam.
FIG. 5 shows a flow chart of purifying crude caprolactam according to the prior art. First, an aqueous solution of crude caprolactam containing 30 to 40% of caprolactam, water, and impurities was delivered via a pipeline 10 to an ion-exchange apparatus set 110 composed of a tower comprising a cation-exchange resin and a tower comprising an anion-exchange resin, where the aqueous solution of crude caprolactam was filtered through the ion-exchange apparatus set 110 to remove the ionic impurities therein. Next, the filtered aqueous solution of crude caprolactam (also referred to as “filtrate” hereinafter) was delivered via a pipeline 12 to a vaporization apparatus 210, where the filtrate was concentrated to obtain a concentrated filtrate containing a high concentration (for example, higher than 80%) of caprolactam. The concentrated filtrate was delivered via a pipeline 14 to a buffer apparatus 220, and then delivered via a pipeline 16 to a distillation apparatus 230, where the concentrated filtrate was further purified, thereby obtaining a final caprolactam product with a high-purity. The final caprolactam product was then recovered through a pipeline 18.
It was necessary to provide an inspection unit at the outlet of the ion-exchange apparatus set to judge whether the filtrate from the ion-exchange apparatus set met the preset inspection standards, thereby assuring that the final caprolactam product always met the standards for caprolactam of industrial grade. The preset inspection standards include the absorbance of the industrial caprolactam at a wavelength of 290 nm (EXT.290) and/or alkaline (ALK), as a standard for inspecting caprolactam-containing filtrate. If the EXT.290 and/or the ALK. of the filtrate exceed the preset upper limits for EXT. 290 and ALK, the quality of the final caprolactam product obtained from the subsequent distillation steps would deteriorate and could not meet the standard of “EXT.290 below 0.05” for caprolactam of industrial grade. Therefore, it was necessary to temporarily stop the filtration operation and replace the ion exchange apparatus before the EXT.290 and/or ALK of the filtrate exceed the upper limits.
However, the inventor of this invention first found that the EXT.290 and/or the ALK of the filtrate, after the aforesaid abrupt rising, would decrease back to the value before the abrupt rising (ie., the values below the preset upper limits) when the ion-exchange apparatus continued running. Thereafter, the EXT.290 and/or the ALK of the filtrate would steadily and slowly increase and remained below the upper limits for a relatively long period of time. In other words, after the EXT.290 and/or the ALK of the filtrate at the outlet of the ion-exchange apparatus abruptly rose and then decreased back, the ion-exchange apparatus could effectively operate for a relatively long period of time and the obtained final caprolactam product that still meets the standards for caprolactam of grade. The currently used ion-exchange resin purification process usually performs the resin replacement process before the EXT.290 and/or the ALK abruptly rise, to avoid producing a final caprolactam product that does not meet the standards. Frequent resin replacement and regeneration process use a large amount of acid, base, and water, and produce a large amount of sewage to be treated.
In view of the above, the inventor of this invention proposes a new system which allows the ion-exchange apparatus to continue running even if the abrupt rising in the EXT.290 and/or ALK of the filtrate occurs, thereby reducing the frequency of replacing and regenerating ion-exchange resin without adversely affect the quality of the final caprolactam product, and reduce the amount of acid, base, and water generated.